1. Field of the Invention
The present invention generally relates to an objective lens actuating device applied to an optical recording/reproducing apparatus and controlling a drive of an objective lens in a focusing direction and/or a tracking direction on a recording surface of a recording medium.
2. Description of the Related Art
In conventional objective lens which is used to form a fine optical spot by condensing a laser beam on a track of an optical disc at a time of recording or reproducing information, an aberration occurs in the optical spot when an optical axis of the objective lens tilts in respect to the optical disc. Thus, problems may be caused during a recording/reproducing operation. Therefore, a tilt of the objective lens is required to be as small as possible. Especially recently, a DVD (Digital Versatile Disk) has been widely marketed to users. And, the DVD for recording is also practically used. In order to improve a higher density of recording information, the objective lens is needed to have a greater number of apertures and then it is more required to prevent the objective lens from being tilted. The main factor causing the tilt of the objective lens is an uneven magnetic field in a magnetic circuit used for a focus actuator.
A conventional objective lens actuating device and focusing will now be described with reference to FIGS. 1A and 1B and FIGS. 2A and 2B. FIG. 1A and FIG. 2A are plan views of the conventional objective lens actuating device. FIG. 1B and FIG. 2B are diagrams showing a relationship between a actuating force and a magnetic flux density.
Referring to FIG. 1A and FIG. 2A, a focusing coil 3 is wound around an outside of a holder 2 holding an objective lens 1. The holder 2 is supported by two pairs of wires 5 so as to be movable in a focusing direction and in a tracking direction. That is, ends of the two pairs of wires 5 in one side are fixed to a wire supporting member 4 and other ends of the two pairs of wires 5 on another side are fixed to both sides of the holder 2.
Each of two magnetic circuits 6 is provided to both sides of the holder 2 in a direction xe2x80x9cAxe2x80x9d and each of air vents 8 is provided to the both sides of the holder 2 in order to insert inner yokes 7 each of which is a part of each of the magnetic circuits 6. Each of the magnetic circuits 6 includes one inner yoke 7, one outer yoke 9, and one permanent magnet 10 fixed by the outer yoke 9.
A magnetic pole surface of each of the permanent magnets 10 is arranged to face to a side surface of the focusing coil 3 in the direction xe2x80x9cAxe2x80x9d. Both of the permanent magnets 10 are magnetized so as to face the same magnetic poles towards each other in the direction xe2x80x9cAxe2x80x9d. Each of the inner yokes 7 is arranged to face to the outer yokes 9 and the permanent magnets 10 inside the focusing coil 3. And the focusing coil 3 is provided at each of gaps 11 between the inner yokes 7 and the permanent magnets 10. A magnetic force as the focus actuating force occurs by a magnetic field within each of the gaps 11 and a current flowing through the focusing coil 3.
Also, two pairs of tracking coils 12 are adhered to both side surfaces of the focusing coil 3 in the direction xe2x80x9cAxe2x80x9d, in which focusing coil 3 is wound around the holder 2 in an optical axis direction of the objective lens 1 in a cylindrical shape, and totally four tracking coils 12 are fixed. In FIG. 1A and FIG. 2A, a gravity center is indicated as a gravity center G of a movable member 13.
As shown in FIG. 1A and FIG. 1B, in a case in which each component is properly arranged, the magnetic field (that is, the focus actuating force), which affects the focusing coil 3 between the gaps 11 of the magnetic circuits 6, shows a symmetric state in that a center of the movable member 13 is stronger and the magnetic field becomes weaker towards each of both sides of the movable member 13. However, as shown in FIG. 2A and FIG. 2B, when a relative displacement between the focusing coil 3 and each of the magnetic circuits 6 occurs, a moment occurs to the movable member 13 since an effect center of the focus actuating force is shifted from the gravity center G. Therefore, the objective lens 1 is tilted.
As a factor of the relative displacement between the focusing coil 3 and the magnetic circuits 6, which displacement causes the moment towards the movable member 13, an error in an initial assembly stage or a move during a tracking operation can be considered.
In order to prevent the tilt of the objective lens 1 caused by the moment occurred by the relative displacement between the focusing coil 3 and the magnetic circuits 6, the Japanese Laid-open Patent Application No. 7-320278 discloses an objective lens actuating device in which a magnetic substance is arranged at a location facing a side surface of the focusing coil 3 in the tracking direction. The Japanese Laid-Open Patent Application No. 9-231593 discloses another objective lens actuating device in that a magnet instead of the magnetic substance is arranged.
However, in such configurations of conventional technologies described above, the magnetic substance or the like is required to be arranged outside the side surface of the focusing coil 3. Thus, these configurations prevent minimizing the objective lens actuating device.
Especially, in a configuration described in the Japanese Laid-Open Patent Application No. 7-320278, the magnetic-field distribution is leveled. However, the moment at the side surface facing the magnetic substance of the focusing coil 3 is increased. Thus, a practical effect cannot be expected in the conventional objective lens actuating device.
It is a general object of the present invention to provide an objective lens actuating device in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide the objective lens actuating device, instead of a large-sized objective lens actuating device, that prevents an objective lens from tilting by canceling a shift from a center of a movable member including the objective lens, and a holder, which shift is caused by a displacement between a focusing coil and a magnetic circuit.
Another specific object of the present invention is to provide the objective lens actuating device, instead of the large-sized objective lens actuating device, that prevents the objective lens from tilting by canceling the shift from the center of the movable member including the objective lens, and the holder, which shift is caused by the displacement caused by a movement during tracking between the focusing coil and the magnetic circuit.
The above objects of the present invention are achieved by an objective lens actuating device including: a holder holding an objective lens in a focusing direction; a focusing coil cylindrically wound in an axis parallel with an optical axis of the objective lens and fixed on the holder; an inner yoke located inside the focusing coil; an outer yoke arranged to partially sandwich the focusing coil and face the inner yoke; a magnetic field generating part generating a magnetic field with the focusing coil in a gap formed by the inner and outer yokes; and an auxiliary magnetic field generating part generating an opposite magnetic field in respect to the magnetic field generated in the gap towards the focusing coil, at both ends of the inner yoke vertical to the focusing direction.
According to the present invention, a special member is not required to mount outside the objective lens actuating device and it is possible to cancel the moment caused by the shift of the focus actuating force by a displacement caused when the focusing coil and the magnetic circuits are assembled, and by the shift of the focus actuating force by a tracking movement in the focusing coil and the magnetic circuits. Also, it is not required to enlarge the objective lens actuating device and it is possible to prevent the objective lens from being tilted.
In the objective lens actuating device, the magnetic field generating part may include a permanent magnet that is fixed on a flat surface of the outer yoke, in which the flat surface is faced to the inner yoke, and that is magnetized in a vertical direction in respect to the flat surface and the auxiliary magnetic field generating part may include both ends of the inner yoke, in which the both ends are formed to be thinner so as to saturate the magnetic field caused by the permanent magnet at the both ends of the inner yoke in a direction perpendicularly crossing the focusing direction. In this configuration, it is not required to provide an additional member such as an auxiliary permanent magnet. Thus, an additional expense is not required to achieve the objects.
Moreover, the above objects of the present invention are achieved by an objective lens actuating device including: a holder holding an objective lens in a focusing direction and in a tracking direction; a focusing coil cylindrically wound in an axis parallel with an optical axis of the objective lens and fixed on the holder; an inner yoke located inside the focusing coil; an outer yoke arranged to partially sandwich the focusing coil and face the inner yoke; a magnetic field generating part generating a magnetic field with the focusing coil in a gap formed by the inner and outer yokes; and an auxiliary magnetic field generating part generating an opposite magnetic field in respect to the magnetic field generated in the gap towards the focusing coil, at both ends of the inner yoke vertical to the focusing direction.
According to the present invention, a special member is not required to mount outside the objective lens actuating device and it is possible to cancel the moment caused by the shift of the focus actuating force by a displacement caused when the focusing coil and the magnetic circuits are assembled, and by the shift of the focus actuating force by a tracking movement in the focusing coil and the magnetic circuits. Also, it is not required to enlarge the objective lens actuating device and it is possible to prevent the objective lens from being tilted.